Many site issues can be caused by corrupt cookies or cache. In order to try to fix these problems, the first step is to clear both cookies and the cache. The Firefox cache temporarily stores images, scripts, and other parts of websites while you are browsing.
Note: This will temporarily log you out of all sites you're logged in to. To clear cache and cookies, do the following:

1. Tap the menu icon located at the top right corner. This is the icon with 3 bars. On older Android devices you'll have to press the hardware menu key and then tap More.
2. Tap Settings.
3. After that, you will be taken to the settings screen. In the settings screen, look under the section Privacy & Security and select Clear private data.
4. You will then be taken to a list of what can be cleared. Select the following 2 for deletion:
   • Cookies & active logins
   • Cache
5. After those have been selected, tap the Clear data button to actually clear the cache and cookies.

Did this help you with your problems? Please let us know!

As you can see in the question description, I already tried that

Have you disabled all addons and themes in firefox then refresh the page?

I have no themes or add-ons once the browser never worked before.

Even trying to load the site through the Private Tab.

PS: it's an Android device, not PC

This may not apply to your device but refer to

• Firefox for Android will not load pages on Android 4.0 (Ice Cream Sandwich)

Im not sure why it isnt loading pages though.

Maybe try the latest version of nightly.

nightly.mozilla.org

Already tried today's version.

How can I report this bug?

bugzilla.mozilla.org